Signal amplifier



March 10, 1953 w. c. MORRISON 2,631,201

SIGNAL AMPLIFIER Filed Feb. 1 25, 1952 INVENTOR ORNEY Patented Mar. 10,1953 SIGNAL AMPLIFIER Wendell C. Morrison, Princeton, N. J., assignor toRadio Corporation of America, a corporation of Delaware ApplicationFebruary 23, 1952, Serial No. 273,096

Claims. (Cl. 179-171) This invention relates to amplifier circuitsemploying degenerative feedback and more particularly to videodistribution amplifiers providing signal energy. for one or two outputcircuits while maintaining constant output signal levels whether one orboth output utilization circuits are connected thereto.

Distribution amplifiers used to provide a single signal at a pluralityof output positions are well known in the art as exemplified by U. S.Patent 2,479,025, issued to J. B. Rudd. Certain prior art amplifiers forvideo frequencies employ degenerative feedback circuits for effecting amore linear frequency response characteristic over a wide range of videofrequencies. In some circuits of this type utilized for distribution attwo output utilization circuits, however, it has been found that becauseof the degenerative feedback networks necessary to provide goo-dfrequency response, the connecting of one load will change I the outputsignal level available at the other output terminal of the distributionamplifier.

When only two stages of amplification are included in the feedback loopof a distribution amplifier utilizing degenerative feedback and designedto drive and to properly terminate a 75 ohm cable, a separate outputtube for each output terminal is generally necessary to provide goodisolation between outputs over a video frequency band. with separateoutput tubes, but

. a common stage of gain in the feedback loop, the

connection of the second load circuit to the amplifier results in anincreased output sign-alto the first load over that obtained when onlyoneload circuit is connected. This results because the feedback signalis decreased and the. gain thereby increases. f

When such a condition exists it is desirable to provide compensatingmeans so that impedance changes at one output terminal do not efiect thesignal amplitude at the other output terminal. It is highly desirable inmany applications of a video distribution amplifier that one circuitconnected to the distribution system should not have its signal voltagevariable with the connection of an additional circuit. For example atelevision transmitter cannot tolerate changes in signal level when amonitoring circuit is connected to the video system.

It is therefore an object of the present invention to provide adistribution amplifier circuit which provides a constant output signallevelat either of two output terminals regardless of the utilizationofthe other output.

In accordance with the present invention the constant signal outputlevel in such a system is accomplished by providing a-compensa tionimpedanc-e network connected between two separate output circuits.Thecompensation network provided has suchcharacteristics that a changein load at one utilization circuit will not efiect any change in signalamplitude at the other utiliza- 1" tion circuit. For a more clearunderstanding of the construction of the invention and its mode ofoperation, the following detailed description may be read in connectionwith the accompanying drawing, in which: 7

Figure 1 is a schematic circuit diagram of a distribution amplifierembodying the invention; and

Figures 2 and 3 are schematic circuit diagrams of compensating network-sconstructed in accordance with the invention for utilization withcircuits of the type shown in Figure l. V

In the embodiment of the invention shown in Figure 1 the distributionamplifier circuit shown will properly terminate either one or two 75 ohmcables which may be selectively coupled to the output terminals 10 andI2. Input signals may be derived from a suitable signal source connectedto the input terminal l4. The particular circuit described is a videoamplifier having a fiat frequency response to at least 4 mo. andproviding good isolation over this frequency band between the outputcircuits connected to termi- Thus a single driver tube It is employedfor amplifying the incoming signal applied to the grid input resistor Itas well as a degenerative feedback wave developed at the oathode inputresistor 20, which will be derived in a manner herein-after discussed.

At the output resistor 220i the input amplifier stage Hi there appears asignal for developing at a single input resistor 25 proper signal inputpotentials for amplification by the separate output distributionamplifier stages 28 and 30. Separate output coupling impedances, neededto provide the fiat frequency response and proper termination for theutilization circuits, are connected to the output connection terminalsl0 and i2 so that two utilization devices, coupled by the hereinbeforementioned ohm cables, may be connected thereto.

At the respective anode circuit resistors 40 and 42 of the distributionamplifier output stages 28 and 30 there is provided .a', sign-a1 ofproper polarity for degenerative feedback when coupled to the L cathoderesistor 20 of the input amplifier stage 116. Such coupling results" intwo stages of amphfica'tion m the degenerative feedback loop which ispartially effective in reducing the output impedance a suitable amount.This isolates the output circuits for proper operation with lowerfrequency video signals but because of reduced amplifier gain at highfrequencies due to shunt capacities does not provide low enoughimpedance over the desired range. If the output impedance were lowenough over the. entire band, load changesat'one terminal would have noeffoot on output signal amplitude at the other terminal.

The degenerative feedback network components are contained within thedotted enclosure '50 and consist of respective series connectedresistors and capacitors and 52, 53 and which provide at the feedbacklead 55 degenerative signal components taken respectively from the anodecircuit of each distribution"amplifier'stage 28 and 30. Furtherdegenerativefeedback is effected in each output'stage 28 and 30 by meansof resistors and 3| for improving low frequency'response. -Consider theconnection of a single 75 ohm load" to th output terminal is. The loadimpedance on the amplifier tube 28 will thereby decrease and a lowerresulting signal potential will therefore permit less degenerativefeedback amplitude through the network 5!, 52'. Accordingly a greatergain is aifordedin'the input stage it} and the'si'gn'aI potential acrossthe load re-. ,si st'or 42 will increase. This effect is apparentwhether a load circuit is connected to terminal I2 or not. Because ofthe decreased load impedance atone output terminal l0 which produces adecreased load for tube 28 the signal voltage thereat tends. to decrease(but is nearly compensated by 'the increased gain), whereas thecorresponding signal voltage at the alternative output terminal I2increases since no load change has occurred for tube 30.

Inaccordance with the invention, however, by connecting the'pr'operimpedance network 50 between the anode circuits of the output amplifierstages 28' and 30, the cross coupling caused by a change of signalvoltage due to connection of a.

. vload circuit may be cancelled outI Therefore a distribution amplifiercircuit for providing two output signals for connection of a pair ofutilization circuits maybe constructed having a constant output signallevel across either utilization circuit without regard to changes in theother utilization circuit.

The" compensation impedance network 60 may .be-ohosen to provide thosecharacteristics desirable any required specific circuit applica- ,tap.connection If and a common feedback network .12 and 13' connectedthereat, resulting in circuit economy. Because of thecapacitance ofelements 12 in Figure 2 or either elements :52

or 54 inthe feedback path of Figure land the capacitance of couplingcapacitor 23 in theffeedback amplification circuit of Figure '1 couplingthe input stage [6 to the output stages 28 and .30, the feedback,andtherefore the output impedance, varies with frequency. The impedanceo'fthe' resistive feedbajckimpedance 10 connected j f'betw eenthe outputimpedancesjhowever does, not -vary with frequency" and therefore thisembodi- 'ment provide a frequencyrse'lective com= e pensation devicewhich is balanced for only a particular frequency. This is desirable inmany cases. However, should broad band compensation be of moreimportance, the impedance network 60 (Figure 1) may comprise capacitiveor inductive components as needed to afiord proper peaking and frequencyselection for compensation over the entire video'bandwith.

Figure 3 shows a further alternative compensating network connectionwherein a purely resistive impedance 60' may be afforded in the circuitof Figure 1. The following table of circult values are illustrative ofan amplifier built in accordance with-the present invention where animpedance network of the type shown in Figure 3 was usedand exactcompensation was efic te @123 m9 R50 43 ohms. R5f and-53 510 ohms. C52and 54 -s. 1,000 mf. "R20 lfio'oh'ms Rl'8 .;l 1' megohm. Rolland 424,500 ohms. CM and 43 mf.:fi R45 "and 4L; lOOohms. R46 and 48 300 ohms.C44 and 49 160 To afford essentially the, same operation with thecircuit of Figure 2', resistor 10 equals 41 ohms center'tapped,capacitor 72 is 1,000'mf. and resistor 13 is 245' ohms.

,It istherefore evident that in accordance with the present inventiontheremay be provided improved distribution amplifier circuits forproviding signal distribution to a plurality of load circults withoutundesirable interaction upon either the coupling, or uncoupling ofdifferent utilization devices or other changes in loading.

If the features of increased ban'dwith and generatorend termination ofthe load circuit are not desired, the output coupling impedance networks(elements '46, d5, 46, H, 48 and 49 of Figure I), may be omitted and thetheory of cross compensation still applied.

Having thus described the invention and its mode of operation, thosenovel features believed descriptiveof the natureof the invention aredescribed with particularity in the appended claims.

What is claimed is:

l. A distribution amplifier circuit for providing .two output signalsfor connection of a pairof utilization circuits, saidamplifier circuitproviding a substantially constant signalievel atone utilization circuitwith changes in .L'Iipedancefof the other utilization circuit 'an'ctcomprisingflin combination, a source of inputsignals comprising 'an' inputamplifier stage, means for simultaneouslyapplyin'g signal energy fromsaid stage tot'wo separate. output amplifier circuits, separate outputcoupling impedance meansconnected respectively in said separateamplifier circuits with terminals connected thereto for selectivelycouplings. pair of utilization circuits, means providing signal energyfeedback from the output couplingimpedances of both said separateamplifier circuits to a suitable connection in said input amplifierstage, and a compensation impedance network connected between both saidoutput coupling impedances having such characteristics that changesofimpedance in either of said utilizfation circuits providesconstantfsignal amplitude atthe other utilization circuit.

compensation impedance network consists of a resistor.

3. A circuit as defined in claim 1 wherein said means providing signalenergy feedback is coupled to a terminal intermediate said compensationimpedance network.

4. An electronic circuit comprising in combination, an input amplifierstage including a tube having both grid and cathode input impedancemeans and output impedance means, a signal source connected to said gridinput impedance means, a pair of output amplifier stages having inputcircuits connected to said output impedance, and having separate outputcoupling impedance means adapted for selective connection to twoutilization devices, degenerative feedback means from each of saidseparate output coupling impedance means to said cathode input impedancemeans, and a compensating impedance network connected between saidseparate output coupling impedance means so proportioned to provide aconstant output signal level at one of said separate output couplingimpedance means upon changes in loading at the other output couplingimpedance means.

5. A distribution amplifier circuit for providing at least two outputsignals for application to a pair of utilization circuits, saidamplifier circuit providing a constant signal level at one utilizationcircuit with changes in impedance of the other utilization circuit andcomprising in combination, a source of input signals comprising an inputamplifier stage, means for simultaneous- 1y applying signal energy fromsaid stage to two separate output amplifier circuits coupled with saidutilization circuits, means providing signal energy feedback from theoutput amplifier circuits to a suitable connection in said inputamplifier stage, and a compensation impedance network connected betweenboth said output amplifier circuits having such characteristics thatchanges of impedance in either of said utilization circuits providesconstant signal amplitude at the other utilization circuit.

The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Rudd Aug. 16, 1949 Number

